basic plc [compatibility mode]

7/29/2019 Basic PLC [Compatibility Mode]

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Description

This training introduces the basic hardware and softwarecomponents of a Programmable Controller (PLC). Itdetails the architecture and basic instruction set commonto all PLCs. Basic programming techniques and logicdesigns are covered. This training describes theoperating features of the PLC, the advantages of thePLC over hard-wired control systems, practicalapplications, troubleshooting and maintenance of PLCs.


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Objectives

At the end of the training the participants should be ableto:

Describe the major components of a common PLC. Interpret PLC specifications. Apply troubleshooting techniques. Convert conventional relay logic to a PLC language. Operate and program a PLC for a given application.

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Course Contents

History of Programmable Controllers Relay Ladder Logic Central Processing Unit Input/Output System Programming and Peripheral Devices Programming Concepts Applications Troubleshooting and Maintenance


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Advantages of PLCs

Less wiring. Wiring between devices and relay contacts are done inthe PLC program. Easier and faster to make changes. Trouble shooting aids make programming easier andreduce downtime. Reliable components make these likely to operate foryears before failure.

INTRODUCTION TO PLCS

PLC Origin

- Developed to replace relays in the late 1960s

- Costs dropped and became popular by 1980s

- Now used in many industrial designs


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Historical Background

The Hydramatic Division of the General MotorsCorporation specified the design criteria for the firstprogrammable controller in 1968

Their primary goal

To eliminate the high costs associated with inflexible,relay-controlled systems.

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Historical Background

The controller had to be designed in modular form, so thatsub-assemblies could be removed easily for replacement orrepair.

The control system needed the capability to pass datacollection to a central system.

The system had to be reusable.

The method used to program the controller had to be simple,so that it could be easily understood by plant personnel.


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Programmable Controller Development

1968 Programmable concept developed1969 Hardware CPU controller, with logic

instructions, 1 K of memory and 128 I/Opoints

1974 Use of several (multi) processors within aPLC - timers and counters; arithmeticoperations; 12 K of memoryand 1024 I/O points

1976 Remote input/output systems introduced1977 Microprocessors - based PLC introduced

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Programmable Controller Development

1980 Intelligent I/O modules developedEnhanced communications facilitiesEnhanced software features(e.g. documentation)Use of personal microcomputers asprogramming aids

1983 Low - cost small PLCs introduced1985 on Networking of all levels of PLC, computer

and machine using SCADA software.


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Programmable Logic Controllers( Definition according to NEMA standard ICS3-1978)

A digitally operating electronic apparatus which uses aprogramming memory for the internal storage of instructionsfor implementing specific functions such as logic,sequencing, timing, counting and arithmetic to controlthrough digital or analog modules, various types of machinesor process.

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Leading Brands Of PLC

AMERICAN 1. Allen Bradley2. Gould Modicon3. Texas Instruments4. General Electric5. Westinghouse6. Cutter Hammer7. Square D

EUROPEAN 1. Siemens2. Klockner & Mouller3. Festo4. Telemechanique


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Leading Brands Of PLC

JAPANESE 1. Toshiba2. Omron3. Fanuc4. Mitsubishi

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Areas of Application

Manufacturing / Machining

Food / Beverage

Metals

Power

Mining

Petrochemical / Chemical


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PLC Size

1. SMALL - it covers units with up to 128 I/Os andmemories up to 2 Kbytes.

- these PLCs are capable of providingsimple to advance levels or machinecontrols.

2. MEDIUM - have up to 2048 I/Os and memories upto 32 Kbytes.

3. LARGE - the most sophisticated units of the PLCfamily. They have up to 8192 I/Os andmemories up to 750 Kbytes.

- can control individual productionprocesses or entire plant.

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Tank Used to Mix Two Liquids

A

B

C

FS

MOTOR

TIMER

FLOAT SWITCH

SOLENOIDS

SOLENOID

1 -MINUTE


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A tank is used to mix two liquids. The control circuit operatesas follows:

1. When the start button is pressed, solenoids A and Benergize. This permits the two liquids to begin filling the tank.

2. When the tank is filled, the float switch trips. This de-energizes solenoids A and B and starts the motor used tomix the liquids together.

3. The motor is permitted to run for one minute. After oneminute has elapsed, the motor turns off and solenoid Cenergizes to drain the tank.

4. When the tank is empty, the float switch de-energizessolenoid C.

5. A stop button can be used to stop the process at anypoint.

6. If the motor becomes overloaded, the action of the entirecircuit will stop.

7. Once the circuit has been energized it will continue tooperate until it is manually stopped.

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Major Components of a Common PLC

PROCESSOR

POWERSUPPLY

I MN OP DU UT L

E

O MU OT DP UU LT E

PROGRAMMINGDEVICE

FromSENSORS

Pushbuttons,contacts,

limit switches,etc.

ToOUTPUT

Solenoids,contactors,

alarmsetc.

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POWER SUPPLY

Provides the voltage needed to run the primary PLCcomponents

I/O MODULES

Provides signal conversion and isolation between theinternal logiclevel signals inside the PLC and the fieldshigh level signal.


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PROCESSOR

Provides intelligence to command and govern the activitiesof the entire PLC systems.

PROGRAMMING DEVICE

used to enter the desired program that will determine thesequence of operation and control of process equipment or

driven machine.

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Programming Device

Also known as:

Industrial Terminal ( Allen Bradley )

Program Development Terminal ( General Electric )

Programming Panel ( Gould Modicon )

Programmer ( Square D )

Program Loader ( Idec-Izumi )

Programming Console ( Keyence / Omron )


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Programming Device

Types:

Hand held unit with LED / LCD display

Desktop type with a CRT display

Compatible computer terminal

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I/O Module The I/O interface section of a PLC connects it toexternal field devices.

The main purpose of the I/O interface is to condition thevarious signals received from or sent to the external inputand output devices.

Input modules converts signals from discrete or analoginput devices to logic levels acceptable to PLCs processor.

Output modules converts signal from the processor tolevels capable of driving the connected discrete or analogoutput devices.


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I/O Module

DC INPUT MODULE

OPTO-

ISOLATOR

IS NEEDED TO: Prevent voltagetransients fromdamaging theprocessor.Helps reduce theeffects of electricalnoise

CurrentLimitingResistor

FROMINPUTDEVICE

USE TODROP THEVOLTAGETO LOGICLEVEL

Buffer,Filter,hysteresisCircuits

TOPROCESSOR

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I/O Module

AC INPUT MODULE

OPTO-ISOLATOR


Rectifier,

ResistorNetwork

FROMINPUTDEVICE

CONVERTS THE ACINPUT TO DC ANDDROPS THE VOLTAGETO LOGIC LEVEL

Buffer,

Filter,HysteresisCircuits

TOPROCESSOR


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I/O Module

DC / AC OUTPUT MODULE

OPTO-

ISOLATOR


FROM

PROCESSOR

TTL

Circuits

AmplifierRELAY

TRIACXSISTOR

TO

OUTPUTDEVICE


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I/O CircuitsDIFFERENT TYPES OF I/O CIRCUITS

1. Pilot Duty OutputsOutputs of this type typically are used to drive high-currentelectromagnetic loads such as solenoids, relays, valves, andmotor starters.

These loads are highly inductive and exhibit a large inrushcurrent.

Pilot duty outputs should be capable of withstanding aninrush current of 10 times the rated load for a short period oftime without failure.


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I/O Circuits

2. General - Purpose OutputsThese are usually low- voltage and low-current and are usedto drive indicating lights and other non-inductive loads. Noisesuppression may or may not be included on this types ofmodules.

3. Discrete InputsCircuits of this type are used to sense the status of limitswitches, push buttons, and other discrete sensors. Noisesuppression is of great importance in preventing falseindication of inputs turning on or off because of noise.

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I/O Circuits4. Analog I/O

Circuits of this type sense or drive analog signals.

Analog inputs come from devices, such as thermocouples,strain gages, or pressure sensors, that provide a signalvoltage or current that is derived from the process variable.Standard Analog Input signals: 4-20mA; 0-10V

Analog outputs can be used to drive devices such as

voltmeters, X-Y recorders, servomotor drives, and valvesthrough the use of transducers.Standard Analog Output signals: 4-20mA; 0-5V; 0-10V


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I/O Circuits

5. Special - Purpose I/O

Circuits of this type are used to interface PLCs to very specifictypes of circuits such as servomotors, stepping motors PID(proportional plus integral plus derivative) loops, high-speedpulse counting, resolver and decoder inputs, multiplexeddisplays, and keyboards.

This module allows for limited access to timer and counterpresets and other PLC variables without requiring a programloader.

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PLC

INPUTS

OUTPUTS

MOTOR

LAMP

CONTACTOR

PUSHBUTTONS


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L1 L2

P. B SWITCH

INPUT MODULE

WIRING DIAGRAM

LADDER PROGRAM

I:2

0

I= Input

Module

slot # in rack

Module

Terminal #

Allen-Bradley 1746-1A16

Address I:2.0/0

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N.O

C

L2 L1

L1

L2

OUTPUT MODULE

WIRING

MOTOR

CONTACTOR

O:4

0CONTACTOR

LADDER PROGRAM

L1 L2

FIELD WIRING

SOLENOIDVALVESLAMPBUZZER


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Discrete Input

A discrete input also referred as digital input is an input that is

either ON or OFF are connected to the PLC digital input. In theON condition it is referred to as logic 1 or a logic high and in theOFF condition maybe referred to as logic o or logic low.

Normally Open Pushbutton

Normally Closed Pushbutton

Normally Open switch

Normally Closed switch

Normally Open contact

Normally closed contact

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OFF

Logic 0

IN

PLC

Input

Module24 V dc

OFF

Logic 1

IN

PLC

InputModule

24 V dc


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IN

PLC

AnalogInput

ModuleTank

Level Transmitter

An analog input is an input signal that has a continuoussignal. Typical inputs may vary from 0 to 20mA, 4 to 20mAor 0 to10V. Below, a level transmitter monitors the level ofliquid in the tank. Depending on the level Tx, the signal to thePLC can either increase or decrease as the level increasesor decreases.

Analog Input

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OUT

PLC

Digital

Output

Module

Lamp

A discrete output is either in an ON or OFF condition. Solenoids,contactors coils, lamps are example of devices connected to theDiscrete or digital outputs. Below, the lamp can be turned ON orOFF by the PLC output it is connected to.

Digital Output


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OUT

PLC

AnalogOutput

Module

An analog output is an output signal that has a continuoussignal. Typical outputs may vary from 0 to 20mA, 4 to 20mAor 0 to10V.

Analog Output

EP

Pneumatic control valve

Supply air

Electric to pneumatic transducer

0 to 10V

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Processor

The processor module contains the PLCs microprocessor,its supporting circuitry, and its memory system.

The main function of the microprocessor is to analyze datacoming from field sensors through input modules, makedecisions based on the users defined control program andreturn signal back through output modules to the fielddevices. Field sensors: switches, flow, level, pressure, temp.transmitters, etc. Field output devices: motors, valves,solenoids, lamps, or audible devices.

The memory system in the processor module has two parts:a system memoryand an application memory.


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Memory Map Organization

SYSTEM

System memory includes an area called the EXECUTIVE,composed of permanently-stored programs that direct all systemactivities, such as execution of the users control program,communication with peripheral devices, and other systemactivities.The system memory also contains the routines that implement thePLCs instruction set, which is composed of specific controlfunctions such as logic, sequencing, timing, counting, andarithmetic.System memory is generally built from read-only memory devices.

APPLICATION

The application memory is divided into the data table area anduser program area.The data table stores any data associated with the users controlprogram, such as system input and output status data, and anystored constants, variables, or preset values. The data table iswhere data is monitored, manipulated, and changed for control

purposes.The user program area is where the programmed instructionsentered by the user are stored as an application control program.

Data Table

User Program

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Memory DesignsVOLATILE.A volatile memory is one that loses its stored informationwhen power is removed.

Even momentary losses of power will erase any informationstored or programmed on a volatile memory chip.

Common Type of Volatile Memory

RAM. Random Access Memory(Read/Write)Read/write indicates that the information stored in thememory can be retrieved or read, while write indicates thatthe user can program or write information into the memory.


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Memory Designs

The words random access refer to the ability of anylocation (address) in the memory to be accessed or used.Ram memory is used for both the user memory (ladderdiagrams) and storage memory in many PLCs.

RAM memory must have battery backup to retain or protectthe stored program.

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Memory DesignsSeveral Types of RAM Memory:

1.MOS2.HMOS3.CMOS

The CMOS-RAM (Complimentary Metal OxideSemiconductor) is probably one of the most popular. CMOS-RAM is popular because it has a very low current drain when

not being accessed (15microamps.), and the informationstored in memory can be retained by as little as 2Vdc.


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Memory Designs

NON-VOLATILEHas the ability to retain stored information when power isremoved, accidentally or intentionally. These memories do notrequire battery back-up.

Common Type of Non-Volatile Memory

ROM, Read Only MemoryRead only indicates that the information stored in memorycan be read only and cannot be changed. Information in ROMis placed there by the manufacturer for the internal use and

operation of the PLC.

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Memory DesignsOther Types of Non-Volatile Memory

PROM, Programmable Read Only MemoryAllows initial and/or additional information to be written intothe chip.

PROM may be written into only once after being receivedfrom the PLC manufacturer; programming is accomplish bypulses of current.

The current melts the fusible links in the device, preventing itfrom being reprogrammed. This type of memory is used toprevent unauthorized program changes.


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Memory Designs

EPROM, Erasable Programmable Read Only Memory

Ideally suited when program storage is to be semi-permanent or additional security is needed to preventunauthorized program changes.

The EPROM chip has a quartz window over a siliconmaterial that contains the electronic integrated circuits. Thiswindow normally is covered by an opaque material, butwhen the opaque material is removed and the circuitryexposed to ultra violet light, the memory content can be

erased.

The EPROM chip is also referred to as UVPROM.

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Memory Designs

EEPROM, Electrically Erasable Programmable ReadOnly Memory

Also referred to as E2PROM, is a chip that can beprogrammed using a standard programming device and canbe erased by the proper signal being applied to the erase pin.

EEPROM is used primarily as a non-volatile backup for the

normal RAM memory. If the program in RAM is lost or erased,a copy of the program stored on an EEPROM chip can bedown loaded into the RAM.


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PLC Operation

Basic Function of a Typical PLC

Read all field input devices via the input interfaces, executethe user program stored in application memory, then, basedon whatever control scheme has been programmed by theuser, turn the field output devices on or off, or performwhatever control is necessary for the process application.

This process of sequentially reading the inputs, executingthe program in memory, and updating the outputs is known

as scanning.

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While the PLC is running, the scanning process includes thefollowing four phases, which are repeated continuously asindividual cycles of operation:

PHASE 2

ProgramExecution

PHASE 3

Diagnostics/Comm

PHASE 4

Output

Scan

PHASE 1

Read Inputs

Scan


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PHASE 1 Input Status scan

A PLC scan cycle begins with the CPU reading the statusof its inputs.

PHASE 2 Logic Solve/Program Execution

The application program is executed using the status ofthe inputs

PHASE 3 Logic Solve/Program Execution

Once the program is executed, the CPU performs

diagnostics and communication tasks

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PHASE 4 - Output Status Scan

An output status scan is then performed, whereby thestored output values are sent to actuators and other fieldoutput devices. The cycle ends by updating the outputs.


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As soon as Phase 4 are completed, the entire cycle beginsagain with Phase 1 input scan.

The time it takes to implement a scan cycle is called SCANTIME. The scan time composed of the program scan time,which is the time required for solving the control program, andthe I/O update time, or time required to read inputs andupdate outputs. The program scan time generally depends onthe amount of memory taken by the control program and typeof instructions used in the program. The time to make a singlescan can vary from 1 ms to 100 ms.

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PLC CommunicationsCommon Uses of PLC Communications Ports

Changing resident PLC programs - uploading/downloadingfrom a supervisory controller (Laptop or desktop computer).

Forcing I/O points and memory elements from a remoteterminal.

Linking a PLC into a control hierarchy containing several

sizes of PLC and computer.

Monitoring data and alarms, etc. via printers or OperatorInterface Units (OIUs).


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PLC Communications

Serial Communications

PLC communications facilities normally provides serialtransmission of information.

Common Standards

RS 232

Used in short-distance computer communications, with the

majority of computer hardware and peripherals. Has a maximum effective distance of approx. 30 m at9600 baud.

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PLC CommunicationsLocal Area Network (LAN)

Local Area Network provides a physical link between alldevices plus providing overall data exchange management orprotocol, ensuring that each device can talk to othermachines and understand data received from them.

LANs provide the common, high-speed data communicationsbus which interconnects any or all devices within the local

area.

LANs are commonly used in business applications to allowseveral users to share costly software packages andperipheral equipment such as printers and hard disk storage.


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PLC Communications

RS 422 / RS 485

Used for longer-distance links, often between several PCsin a distributed system. RS 485 can have a maximumdistance of about 1000 meters.

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PLC CommunicationsProgrammable Controllers and Networks

Dedicated Network System of Different Manufacturers

Manufacturer Networ k

Allen-Bradley Data Highway

Gould Modicon Modbus

General Electric GE Net Factory LAN

Mitsubishi Melsec-NET

Square D SY/NET

Texas Instruments TIWAY


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Specifications

Several factors are used for evaluating the quality andperformance of programmable controllers when selecting aunit for a particular application. These are listed below.

NUMBER OF I /O PORTS

This specifies the number of I/O devices that can beconnected to the controller. There should be sufficient I/Oports to meet present requirements with enough spares toprovide for moderate future expansion.

Selecting a PLC

Criteria

Number of logical inputs and outputs. Memory Number of special I/O modules Scan Time Communications Software


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A Detailed Design Process

1. Understand the process

2. Hardware/software selection

3. Develop ladder logic

4. Determine scan times and memory requirements

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Specifications

OUTPUT-PORT POWER RATINGS

Each output port should be capable of supplying sufficientvoltage and current to drive the output peripheral connectedto it.

SCAN TIME

This is the speed at which the controller executes the relay-ladder logic program. This variable is usually specified as thescan time per 1000 logic nodes and typically ranges from 1 to200 milliseconds.


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Specifications

MEMORY CAPACITY

The amount of memory required for a particular application isrelated to the length of the program and the complexity of thecontrol system. Simple applications having just a few relaysdo not require significant amount of memory. Program lengthtend to expand after the system have been used for a while. Itis advantageous to a acquire a controller that has morememory than is presently needed.

PLC Status Indicators

Power On

Run Mode

Programming Mode

Fault


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Troubleshooting

1. Look at the process2. PLC status lights

HALT - something has stopped the CPURUN - the PLC thinks it is OK (and probably is)ERROR - a physical problem has occurred with the PLC

3. Indicator lights on I/O cards and sensors4. Consult the manuals, or use software if available.5. Use programming terminal / laptop.

List of items required when working with PLCs:

1. Programming Terminal - laptop or desktop PC.2. PLC Software. PLC manufacturers have

their own specific software and license key.3. Communication cable for connection from Laptop

to PLC.4. Backup copy of the ladder program (on diskette, CDROM,

hard disk, flash memory). If none, upload it from the PLC.5. Documentation- (PLC manual, Software manual, drawings,

ladder program printout, and Seq. of Operations manual.)


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Examples of PLC Programming Software:

1. Allen-Bradley Rockwell Software RSLogix5002. Modicon - Modsoft3. Omron - Syswin4. GE-Fanuc Series 6 LogicMaster65. Square D- PowerLogic6. Texas Instruments Simatic6. Telemecanique Modicon TSX Micro

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PROGRAMMING

Normally Open(NO)

Normally Closed(NC)

Power flows through these contacts when they are closed. Thenormally open (NO) is true when the input or output status bitcontrolling the contact is 1. The normally closed (NC) is true

when the input or output status bit controlling the contact is 0.


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Coils

Coils represent relays that are energized when power flows tothem. When a coil is energized it causes a correspondingoutput to turn on by changing the state of the status bit controllingthe output to 1. That same output status bit maybe used to controlnormally open or normally closed contact anywhere in the program.

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Boxes

Boxes represent various instructions or functions that are

Executed when power flows to the box. Some of theseFunctions are timers, counters and math operations.


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AND OPERATION

Each rung or network on a ladder program representsa logic operation. In the rung above, both inputs A and Bmust be true (1) in order for the output C to be true (1).

Rung

A B C

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OR OPERATION

In the rung above, it can be seen that either input A or Bis be true (1), or both are true, then the output C is true (1).

Rung

A

B

C


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NOT OPERATION

In the rung above, it can be seen that if input A is be true (1),then the output C is true (0) or when A is (0), output C is 1.

Rung

A C

basic plc [compatibility mode]

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